AIODERN TIMES. The develomnent of our pres ent knowledge of physics is more fully described under the separate headings of its various sub jects; namely, acoustics, heat, light, electricity, and magnetism. It should he sufficient in this place to refer to a few' names in connection with each of the subjects. Our accepted ideas in regard to the nature of beat and heat effects are due to the researches and theories of Black, Rum ford, Davy, Carnot, Clausius, and Joule. In light we owe most to Young, Fresnel. Fraunhofer, Kirchhoff, and Stokes. In the subjects of elee trieity and magnetism a long list of names should he mentioned, but among these those of Volta, Cavendish, Ampere, Oersted. Faraday, Henry, Gauss, Weber, Maxwell, and Hertz are easily the most notable. To Helmholtz alone, it is only fair to say, all of our accepted ideas of the theory of sound are due. The names of no living men have been included in tl.e above summary, but any such list would he incomplete without the addition of the names of Kelvin, Rayleigh, Lo renz, J. J. Thomson, and RiMtgen, who have con tributed so much to the most recent ideas in regard to the fundamental concepts of nature.
Previous to the nineteenth century there were three great principles of physics which were firmly established and which deserve to be called laws of nature. These were. the law of gravi tation, the principle of the conservation of ter, and that of the conservation of momentmn. Some doubts have recently arisen as to whether the weight of a definite quantity of matter re mains unchanged as its state is altered, but there seems to he no reason for doubting the principle of the conservation of matter itself. The most notable fact in the history of physics during the nineteenth century was the development of the principle of the conservation of energy and its application to all fields of science. This great development was due largely to the efforts of J. R. Helmholtz, and -Tonle. Other great principles, such as the second law of ther modynamics, were expressed in words, and will be found discussed in other places. The kinetic theory of matter was also elaborated and estab lished on a firm basis.
Beginning with the discovery of the X rays by Rdntgen in 1895 and of the radiation from uranium by H. Becquerel in 1896. there has been a progress in physics and in our knowledge of the properties of matter which has been one of the most striking facts in the whole history of science. The point of departure in our new
knowledge comes from the epoclr-making discov ery by J. J. Thomson that in their passage through air both the X rays and those from uranium make the air a conductor for electric currents. This discovery led at once to investiga tions by observers the world over in regard to the nature of the ionization of air and of the radiations which produce it. It was soon found that many substances other than uranium had this same power of emitting radiations which would ionize a gas; such were thorium, and the newly discovered elements, radium and polo nium. These substances are said to be 'radio active;' and it was observed that nearly every body in nature was `active' to a greater or less degree. The properties of the radiations them selves were carefully studied and analyzed, and their properties other than that of ionization were investigated. There are always present in the radiations from such bodies both positively and negatively charged particles of matter whose velocities vary within wide limits. When the velocities of the negatively charged particles as produced in various ways were studied, a most remarkable fact was observed. Attention had been called many years ago by J. J. Thomson to the fact that if a charged sphere were moving rapidly it would have an effective inertia greater than that which would be observed in ease it. were uncharged. En other words, an electrical charge in motion has an apparent mass quite apart from that of the matter carrying it: and formulas have been deduced by Thomson, Bea vi side, and Abraham connecting this mass of the charge with the quantity of charge, its velocity, and its acceleration. It was observed in the case of the negatively charged radiations above referred to that their apparent mass was exactly that which would lie caleulated from the formuhe on the assumption that the mass was due entire ly to the moving charge. This discovery renewed at once the theories of matter which had been advanced before, in accordance with which the inertia of matter is a property clue to the motion of an electric charge. The name 'electron' has been given these charges when considered apart from matter, and a most interesting discussion of their properties and of the theories of matter based upon these is given in a series of papers by Sir Oliver Lodge in the Electrician (London) during the winter of 1:102-03.